黃酸樹膠之溶凝膠轉換和液晶相變化研究

The Syudy of Sol/Gel Transition and Liquid Crystal Transition in Xanthan Gum

黃美鳳(HUANG MEI FENG)

指導教授 ： 芮祥鵬

國立臺北科技大學/工程學院/有機高分子研究所/碩士(2006年)

摘要

黃酸樹膠是由Xanthomonas campestris菌株發酵產生的一種具有特殊流變性質與商業價值的天然高分子量之細胞外多醣體(extracellular polysaccharude)，黃酸樹膠水溶液的黏度是非牛頓流體(non-Newtonian liquid)具有高假塑性(pseudo-plastic)性質，其黏度則與溫度、剪切速度(shear rate)及時間都相關。由於它的諸多特性，因此常使用於食品、紡織、化妝品工業。
由於黃酸樹膠分子間的交互作用力其流變性往往易受到溫度、pH、鹽類或其它條件所影響，且於低濃度和高濃度下有不同之性質，故本研究首先利用SR5流變儀試著找出其溶膠/凝膠狀態轉換臨界濃度，並分別於溶膠和凝膠狀態下做流變性質探討，結果發現於溶膠態添加鹽類黏度下降而凝膠狀態下鹽類的添加反而使得黏度上升。
黃酸樹膠於凝膠狀態尚有液晶相的特性，依它的形成機制為液向型(lyotropic)液晶而液向型液晶表現方式是由濃度控制，依分子排列方式屬於向列型(nematic)液晶。由於黃酸樹膠的結構會隨著加熱而發生構形轉變，並由低溫的異向性加熱到一定溫定後形成同向性。故本實驗以偏光顯微鏡及加熱板觀察不同濃度且分別再添加鹽類及調整pH值之黃酸樹膠，試著找出其液晶相溫度範圍，另外再藉由流變儀探討其黏度、流動活化能和滯後性之性質，由結果發現不論是黏度或滯後性其在異向區會受結構影響而在同向區則受溫度影響。
黃酸樹膠經由甲醛進行醛化改質後，由於其主鏈結構改變了，使得液晶相明顯變弱了，且於液晶區其黏度或滯後性變化都不大。

關鍵字

流變學 ； 黃酸樹膠 ； 溶膠 ； 凝膠 ； 液晶

並列摘要

Xanthan gum is a natural high molecular weight extracellular polysaccharide with exceptional rheological properties produced by the bacterium Xanthomonas. Xanthan gum solutions are non-Newtonian and highly pseudoplastic. The viscosity of Newtonian fluids depends on temperature, shear rate, time. Due to its exceptional rheological properties this polymer is actually employed in many industrial processes
such as food,textile,and cosmetics.
The effects of temperature, pH value and salt on viscosity of xanthan solutions are of great importance. The xanthan gum have different properties in low and high polymer concentration. The rheometry SR5 was performed in this study for trying to find the sol/gel transition and measuring the viscoelasticities at high and low polymer concentration range respectively. Accroding to the result we found that the effects of the addition of salt depend on the xanthan concentration. At sol status, the addition of salt reduces the viscosity of the preparation; however, at gel condition , the salt is seen to increase the viscosity of the formulation.
Xanthan gum in gel condition still have the properties of liquid crystal. Xanthan gum, which possesses lyotropic properties.and it was confirmed by optical studies that showed samples in this region to consist of a nematic liquid crystalline phase dispersed in an isotropic phase. The structure of xanthan gum have changed with heating, it is anisotropic in low temperature , but it is become isotropic with heating until the special temperayure .The polarized optical microscopy was performed in this study for trying to find the temperature range of xanthan gum’s liquid crystal. We also discuss the activation energy of flow and hysteresis. Accroding to the conclusion, the viscosity and hysteresis are influence by structure in anisotropic , but in isotropic are influence by temperature.
Xanthan gum is modified with formaldehyde, the structure of main chain have changed. The liquid crystal phases are unapparent , and no matter the viscosity or hysteresis in anisotropic are not changed a lot

並列關鍵字

rheology ； xanthan gum ； sol ； gel ； liquid crystal

參考文獻

【17】林怡雅, 探討生物聚醣三仙膠在生物吸附程序的應用(Evaluate the Bacterial polysaccharide of Xanthan Cum for the Application of Biosorption Processes) ,中原大學碩士論文, 2005, 6 月

【8】劉士榮編著,高分子流變學,滄海書局,2005

【21】施寬裕, 側鏈鐵電性液晶聚環氧丙烷之合成及性質研究 ( IX )

【20】李孟良, 生物可分解性聚合物摻和系統之結晶行為研究(A Study of Crystallization Behavior in PLA Compounded System), 台北科技大學有機高分子碩士論文, 2005, 6 月

【19】張佳揚, 液晶電流變行為之研究(The Study of Electrorheological Behavior of Liquid Crystal) 台北科技大學有機高分子碩士論文, 2005, 6 月